北大: https://www.math.pku.edu.cn/teachers/lidf/docs/Rbook/html/_Rbook/prog-type-char.html 画图: https://bookdown.org/xiangyun/msg/foreword.html R语言实战医学统计: https://ayueme.github.io/index.html 祥云r语言笔记 https://bookdown.org/xiangyun/notesdown